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Oligocene trondhjemitic dikes in the Austroalpine basement of the Pfunderer Berge, Südtirol - level of emplacement and metamorphic overprint

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany

^* Corresponding author, e-mail: bernhard.stoeckhert{at}ruhr-uni-bochum.de

Trondhjemitic to tonalitic porphyric dikes in the Austroalpine basement of the Pfunderer Berge, Südtirol, are genetically related to the adjacent Oligocene intrusions, the Rieserferner pluton to the east, and the Rensen pluton to the west, both dated at about 30 Ma. Their phase assemblage, mineral composition, and microstructure are investigated to gain insight into the level of emplacement, and the conditions and time scales of metamorphic overprint and deformation. For samples with uniform whole rock chemistry, the phenocryst assemblage and microstructure of the dikes change from east to west. In the east, the dikes contain phenocrysts of igneous garnet, but no muscovite. In the west, the dikes contain phenocrysts of muscovite, but no igneous garnet, while fine-grained post-magmatic garnet has formed at subsolidus conditions. The igneous garnet in the eastern dikes crystallised at temperatures of about 800 ± 50°C, as indicated by biotite-garnet exchange thermometry. The eastern dikes underwent only slight subsolidus deformation at temperatures near 300°C and the original magmatic microstructure is largely preserved. In contrast, the western dikes are foliated and show intense subsolidus deformation of quartz in the dislocation creep regime. The composition of the garnet-phengite-plagioclase-biotite assemblages of the fine-grained matrix indicates crystallisation at temperatures of about 500 to 600°C and pressures of about 0.5 to 0.6 GPa, which corresponds to the minimum pressure required to stabilise the igneous muscovite at about 700°C. Most likely crystallisation time scales in magma chambers on the order of a few hundred years are obtained from kinetic modelling, with dike emplacement being possibly triggered by a magma mixing event. In contrast, subsolidus cooling of some dikes emplaced at deeper levels spanned millions of years. The results show that the eastern dikes were emplaced at a shallower crustal level and within a cooler country rock, compared to the western dikes - the amount of uplift and erosion since the contemporaneous intrusion at about 30 Ma increasing from about 10 km in the Pfunderer Berge to about 15 to 25 km in the Rensen area in the west.

Key-words: trondhjemite, dike, level of emplacement, geothermobarometry, thermal history, diffusion in garnet, Eastern Alps.

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